Hydrogen peroxide has an oxidizing power such that it exhibits strong bleaching and germicidal actions. Therefore, hydrogen peroxide is utilized as a bleaching agent for, for example, paper, pulp, and fibers, or as a germicide. Further, hydrogen peroxide is an important industrial product widely used in oxidation reactions including epoxidation and hydroxylation.
Further, hydrogen peroxide is used in the semiconductor industry, specifically used in, for example, cleaning of the surfaces of semiconductor substrates and the like, chemical polishing of the surfaces of copper, tin, and other copper alloys, and etching for electronic circuit. Hydrogen peroxide produces only water and oxygen as decomposition products, and hence is considered important from the viewpoint of green chemistry, and has attracted attention as a substitute material for a chlorine bleaching agent.
Conventionally, as a method for producing hydrogen peroxide, for example, an anthraquinone method, an electrolytic method, and a method using oxidation of isopropyl alcohol have been known, and the anthraquinone method is industrially mainly employed. The anthraquinone method, however, is a method comprising many steps, such as hydrogenation of anthraquinone, oxidation by air, extraction with water of the formed hydrogen peroxide, further, purification and concentration. Thus, this method requires high plant investment and uses a large amount of energy, and further causes emission of an organic solvent used for dissolving anthraquinone to the atmosphere, and hence is not an ideal method for producing hydrogen peroxide.
As a method for solving the above problems, there is a method for directly producing hydrogen peroxide from oxygen and hydrogen in a reaction medium using a catalyst. For example, there has been proposed a method in which hydrogen and oxygen are contacted with a solid catalyst containing gold, platinum, or palladium as a metal component in a liquid phase in the presence of water, an acid, and a non-acidic oxygen-containing organic compound to produce hydrogen peroxide, and it is known that hydrogen peroxide is formed by this method in a certain yield (patent document 1).
In the method in which hydrogen peroxide is directly produced from oxygen and hydrogen using a noble metal catalyst, the catalyst functions also as a catalyst for decomposition of hydrogen peroxide, and therefore decomposition of the formed hydrogen peroxide simultaneously occurs. For this reason, in such a method, a compound for inhibiting the decomposition is generally used, and patent document 1 proposes that, in addition to the above-mentioned non-acidic oxygen-containing organic compound, ions such as sulfuric acid ions, chlorine ions, or bromine ions are present in a liquid phase of the reaction medium.
Patent document 2 discloses a method for catalytically producing hydrogen peroxide from hydrogen and oxygen in a reaction medium using a platinum group metal catalyst supported on an oxide carrier. This patent document reports that water is generally preferred as the reaction medium, and that, for inhibiting decomposition of the formed hydrogen peroxide, for example, an aqueous hydrochloric acid solution, an aqueous hydrobromic acid solution, an aqueous phosphoric acid solution, or an aqueous sulfuric acid solution can be preferably used, particularly, an aqueous hydrochloric acid solution or an aqueous hydrobromic acid solution can be preferably used. In addition, there is a description that, instead of the aqueous hydrochloric acid solution, a combination of an aqueous solution mixture of sodium chloride, potassium chloride or the like as a chloride ion component and sulfuric acid, phosphoric acid or the like as a hydrogen ion component, can be preferably employed. Further, there is a description that, instead of the aqueous hydrobromic acid solution, a combination of an aqueous solution mixture of sodium bromide, potassium bromide as a bromide ion component and sulfuric acid, phosphoric acid or the like as a hydrogen ion component, can be preferably employed.
Patent document 3 proposes a method for directly producing an aqueous hydrogen peroxide solution from hydrogen and oxygen in an agitator type reactor, wherein hydrogen and oxygen are separately in the form of small bubbles and preliminarily acidic by adding an inorganic acid thereto, and the molar ratio between the amounts of hydrogen and oxygen introduced is constant. This patent document has a description showing that the aqueous reaction medium can contain a stabilizer for hydrogen peroxide (e.g., a phosphonate or tin) and an inhibitor of decomposition of hydrogen peroxide (e.g., a halide). Further, this patent document has a description showing that, among the halides, a bromide is an especially preferred inhibitor of decomposition, and a combination with free bromine (Br2) is advantageously used.
Patent document 4 discloses a method for producing an organic hydrogen peroxide solution or an aqueous organic hydrogen peroxide solution by a direct synthesis method, wherein a non-explosive gas mixture containing hydrogen and oxygen and a liquid reaction medium are passed through a fixed bed comprising a mixture containing a noble metal catalyst. Further, this patent document discloses that the liquid reaction medium contains a strong acid and a halide.
Patent document 5 discloses a direct synthesis method of an aqueous solution of hydrogen peroxide from hydrogen and oxygen in a three-phase system using a heterogeneous catalytic action, wherein the reaction proceeds directly on the surface of a solid heterogeneous catalyst in a particulate form suspended in a liquid aqueous phase, and the catalyst comprises pure palladium or a metal compound selected from combinations of palladium and at least one noble metal other than palladium. Further, this patent document discloses that, in this method, the metal compound is supported on a carrier comprising at least one compound selected from zirconium dioxide and superacidic zirconium dioxide, and that the liquid aqueous phase contains therein bromide ions at a concentration of 0.05 to 3 mmol/liter, based on the aqueous phase, and has a pH in the range of from 0 to 4.